The present invention relates to a method for the production of Nylon 6 which employs a catalyst composition comprising an alkali metal hypophosphite and an organic phosphite. More particularly, the present invention relates to a method which utilizes a catalyst composition comprising an alkali metal hypophosphite as the primary catalyst and an organic phosphite as a cocatalyst, to increase the rate of polymerization of Nylon 6.
Nylon 6 has been widely used as fibers and engineering plastics. In its application as engineering plastics, Nylon 6 can be used for making automobile parts, electric and electronic components, electric wire coatings, tool boxes, and coasters. As fibers, Nylon can be used in making fishing nets, carpets, tires and brushes (including toothbrushes, cosmetic brushes and grinding brushes) as well as stockings.
Nylon 6 can be synthesized using the conventional continuous reaction process, such as the V. K. Tower process, or the reactive extrusion process, which was developed only recently. In reactive extrusion process to synthesize polymers, the extruder is used as a mini-reactor. First, monomers, co-monomers and/or prepolymers are charged into the extruder. The reactants react inside the extruder to produce the polymer material, which is then extruded from the extruder to provide the final product.
The reactive extrusion technology has the following advantages: First, it provides a high area-to-volume ratio for excellent heat transfer. Second, it provides excellent mixing action during the polymerization reaction. This is particularly advantageous for highly viscous fluids, which can also be transported through the reactor. Third, it can be adapted to provide multiple temperature zones, each having a distinct temperature. Fourth, the reactive extruder can be adapted to provide multiple exhaust or vacuum sections, each vacuum section can have the same or different of vacuum. Fifth, it involves a continuous reaction-extrusion process. Sixth, the resident time involved in the process is very short; therefore, the productivity can be very high per unit time. Seventh, the reactive extrusion process requires a much simpler apparatus than most other conventional commercial reactors. Finally, with the reactive extrusion process, the composition of the feed monomers can be conveniently varied; this greatly enhances the utilization efficiency of the capital investment, and is particularly advantageous for production lines that make relatively small quantities of a large variety of products.
With the above mentioned advantages, the reactive extrusion technology potentially can become one of the best methods to produce polymers and copolymer. However, when the reactive extrusion technology is applied to the polymerization of Nylon 6, current catalyst systems do not provide fast enough reaction rate to take full advantage of this evolving technology. The reaction must be very fast so that the polymerization reaction to make Nylon can be completed in the extruder before they are extruded.